1. Field of the Invention
The present invention relates to a focus detecting apparatus for detecting a focus state of a taking (i.e., photographing) lens of a camera or the like, with respect to an object to be photographed.
2. Description of Related Art
In a known focus detecting apparatus of a single lens reflex camera or the like, bundles of rays transmitted through different portions of an exit pupil of a taking lens are converged onto a pair of line sensors by a re-imaging lens (image reforming lens) of a focus detecting optical system. The focus state of the taking lens can then be detected in accordance with the relationship between outputs of the line sensors. The principle of focus detection by the focus detecting apparatus is disclosed, for example, in U.S. Pat. No. 4,636,624.
A focus detecting apparatus having an off-axis detecting area (i.e., a detecting area that is deviated from an optical axis) is a known apparatus for detecting the focus state of a taking lens with respect to an object located at a position other than the center of an image plane.
However, if the off-axis detecting area is significantly deviated from the optical axis, vignetting of the focus detecting optical system pupil may occur, for example, when the position or size of the exit pupil of the taking lens changes as a result of a lens change (i.e., when using an interchangeable lens) or when a zooming operation is effected. In such cases, the area of the focus detecting optical system pupil, through which the bundle of lo rays is transmitted, is reduced. As a result, the applicability of such an automatic focus detecting system to an interchangeable lens is limited or restricted.